This invention relates to underground electrical conduit, and more particularly to underground electrical conduit having a plurality of channels or ducts for receiving electrical cables or wires. Still more particularly this invention relates to underground electrical conduit having a plurality of channels or ducts for receiving electrical cables including communications cables and the like wherein the channels are formed within the conduit by partition walls which also serve to strengthen and support the exterior walls of the conduit.
Where electrical cables and particularly communications cables are placed underground, conduit is frequently employed to permit the cables to be readily installed and removed for repair and replacement. Conventional underground electrical conduit is generally formed of terra cotta, concrete, fire clay or similar material to provide a strong, rigid structure having the ability to withstand heavy external loading when buried. These conduit are generally rectangular in cross-section and contain a plurality of independent channels or ducts for receiving electrical cable.
Since the density of the materials employed in their construction is quite high, these prior art conduit are made in short sections to reduce weight and thereby facilitate handling and installation. The sections are then joined in the field to form continuous lengths of conduit. Joints between the sections must be carefully made to ensure alignment of the individual ducts and tightly sealed with mortar, pitch or other sealant to prevent the intrusion of subsurface moisture. Installation of these short conduit sections is thus time consuming, and each of the many joints is a potential source of problems due to sealant failure and consequent leakage. Further, though rigid and resistant to compressive loads, these prior art conduit are fragile and breakage during the transporting and installing of the sections is frequently a problem.
More recently a plastic conduit of rectangular cross-section has been introduced to the field. These plastic conduit, injection molded from structural foam, are also generally made in short sections and joined in the field to form a continuous length of conduit. The injection-molded structural foam conduit has the advantages of being lighter in weight than an equivalent concrete conduit and of resisting breakage during installation. However, the outer walls of plastic conduit must be made thick enough to withstand external compressive loads without being crushed or deflected to a degree that will distort and constrict the individual ducts. The need for increased wall thickness in turn increases the amount of plastic material employed, thereby increasing the overall weight and cost of these conduit.
Still more recently, as was disclosed in U.S. Pat. No. 3,941,157, an extruded plastic conduit was introduced for use as underground conduit. The extruded conduit includes longitudinal ribs to reinforce the external walls and provide improved rigidity and load bearing characteristics, as compared with the injection molded conduit previously described. Although the conduit may be extruded in a variety of cross sections including rectangular and circular, the use of external reinforcing ribs to resist bending and crushing raises the volume of plastic material used and increases the complexity of the extrusion process.
The prior art has thus not provided an extruded conduit designed with the strength and rigidity to resist crushing and deflection under compressive loads without resorting to thick external walls and/or the use of reinforcing ribs.